Monster Inside Me
by Coolcatstar
Summary: Leo gets hit with a dart full of this weird chemical. None of the brothers thought it did anything. But little do they know, that Leo would slowly lose his sanity and joins the foot.
1. Chapter 1

The turtles were fighting the footbots this night, but something was different about these seemed larger and bulkier. This bothered the leader. He didn't like this one bit. He took moments to look around just to make sure that his brothers were ok. Donatello was holding his own quite well and by the looks of it, noticed the footbots new look. Michelangelo on the other hand, did not see this change, and didn't even seemed fazed by this. Leo decided to leave him alone about that because he seemed to be fighting pretty well without seeing the size difference of the stupid bots. Raphael looked like he didn't even care about the change, and destroyed the bots one my one.

Leonardo was taken out of his thoughts when he heard the sound of a sword cutting the air coming right at him. He quickly blocked the sword from going through his skull and kicked the bot away. Little did any of them know, Karai was sitting on a billboard stand a little bit away. She took out her blow dart and targeted on of the turtles. Leonardo. She waited for the an opening to shoot the dart. 'I hope he's worth it..' she thought. She found the opening and shot the dart, which successfully lodged itself in Leo's bicep, and caused Leo to shout out in surprise and pain. She then signaled the bots to retreat and disappeared along with them.

Leo was holding his arm as he winced in pain. He dart was small, but very painful. His brothers came running up to him "Leo are you ok?" Donnie was the first to speak.

"Yea, I guess…" Leo said slowly.

He noticed the dart in Leo's arm and took it out a little too quickly which caused Leo to yelp. "sorry Leo." Don examined the dart and realized that there was something in it! He looked up "Leo there was something in this dart, do you feel any different?"

"No, my arm's just sore from the dart, little exhausted from the fight, but other than that I feel..." Leo wobbled and almost fell if it wasn't for Raph, who caught him "...Fine… ugh.. now everything hurts… Mostly my head…" Leo moaned.

"It's ok bro, easy there.." Raph said suddenly getting really concerned. "Donnie, anything you can do to help him?"

"Not here, we gotta get to my lab so I can analyze this dart and take car of Leo. Raph can you help Leo home?" Donnie asked as he started to walked toward the fire escape.

"Yea sure. Mike wanna help?" Raph turned to his little brother realizing he hadn't heard the goofball say anything. He tried to get a hold of Leo better.

Mikey looked at Raph. "Uh, sure.." He went over to Raph and grabbed leo's other side and helped his brother back to the lair.

* * *

The guys had come back to the lair after a half hour of walking. Leo had passed out from the exhaustion and pain. Raph took over on holding Leo and ran as fast as he could with Leo to Don's lab.

"Raph, put Leo on the cot over there and help me gather everything I will need for analyzing the dart and for tending Leo." Donnie instructed not bothering to look at his brother, as he started searching.

Raph nodded and brought his older brother to the cot, then went to go help Donnie. Mikey came into the lab at this time and sat down next to Leo's unconscious form. He looked down at his brother, then grabbed his hand and waited for Leo to wake up. A few moments later, Leo woke up.

Leo slowly opened his eyes and looked around trying to remember how he ended up in the lab. Then he remembered. He sighed. The sigh was just enough to catch the orange clad brother's attention. Mike looked down.

"Leo! How do you feel?" Mike eagerly asked.

"Everythings sore, headache, didn't I say this earlier?" Leo asked. He was having verbal Déjà Vu.

"Oh yea, sorry…" Mikey said

"It's ok Mike, you're just worried. It's fine" Leo slurred the last two words. 'Wow, guess I'm tired...' he thought.

By this time, the other two siblings joined them. "Leo, I see you're up! here have these painkillers for your headache, and the soreness you have should wear off over time. Right now you look really tired. Raph,please take him to his room so he can rest." Donnie instructed again.

Once again, Raph nodded and helped Leo to his room. Once in his room he led Leo to his bed and layed him down in it. "Now you heard Dr. Donnie, you need rest so no getting up. Got it?" Raph questioned his brother. Leo nodded. "Good, see ya later bro." Raph exited Leo's room.

* * *

It was around midnight when Leo's head shot off the pillow. His head hurt a lot and he felt his sanity coming out of him. He began to panic and ran to the lab. As he got to the door of the lab, he lost control for a moment. He spotted his brother's sleeping form at his desk. Leo ran towards his brother, but took hold of his body again before he could hurt his brother.

His eyes widened at the realization that if he stays any longer, he would hurt someone he loves. He backed away from his sleeping brother and stopped at the mirror that Donnie keeps in the lab, and looked into it. Leo looked in horror as he noticed that his eyes were starting to glow red! Leo ran out of the lab, and out of the lair with his swords. 'I HAVE to get out of here!' he said. 'Must stay away. Must stay away…" he repeated. He was running away from his brothers, father figure, friends, ….. home…. 'yes. this is for the best…' he thought with one tea streaming down his face 'for the best' and continued running down the dark sewers.


	2. Chapter 2

Morning came and so far no one has noticed that Leo wasn't there. They all thought he was meditating in his room, so they decided to leave him alone. But then lunch time came and Leo was still no where to be decided to go check up on him. He walked over to Leo's door and knocked.

"Leo? Hey it's lunch time, you comin out or not?" he asked. He waited, and after a minute there was still no response from the other side. "Leo? Did you hear me?" he waited again. Still no response. Raph got irritated and started to walk in. "Leo?" he said a he was opening the door. When he got in the room, he found it empty. Now Raph was getting worried. 'Where the shell is he?'

He walked out and went to the kitchen to talk to his brothers. As he entered, he found Donnie and Mikey staring at him. "Hey Raph, I thought you went to go get Leo. Where is he?" Mikey asked looking around Raph to see if Leo was behind him.

"Yeah, about that-" Raph was cut off before he was able to tell them.

"He's ok, isn't he?" Donnie asked eagerly.

"I don't know, he's not in his room, shell, he's not in the lair and his katana are gone as well!" Raph explained.

His younger brothers looked at Raph in surprise. "Wait, what do you mean 'he's not in the lair'?" Donnie at this point really wanted to know where his oldest brother. "He WAS here last night, why would he leave all of a sudden?"

"I don't know! All I know is that he ain't here and I guess left last night! He couldn't have possibly leave this morning without any of us seeing him?" Raph remarked. "We gotta go find him. We should check the sewers now, and go look topside tonight."

"Yea! Come on let's go!" Mike bounced out of his seat. Right then, Master Splinter came out of nowhere, like he always does, and spoke up.

"Where are you going my sons?" he questioned them.

"Leo's missing and we need to find him. He ran off last night while we were sleeping and we need to find out why! So were going to look through the sewers and try to find him." the red explained.

"Hmm. I find it quite strange that your brother would suddenly leave like that. very well, find your brother, and bring him back safely." Splinter said slowly.

"Hai sensei." the three said together, then grabbed their weapons, and ran out of the lair.

Leo had found a tunnle two miles away to sleep in. It was the driest place he could find, but still pretty damp. He woke up and his head was still hurting. 'heh, I bette keep going before my brothers find me..' he thought to himself. Then, he faintly heard his name being called. 'Crap…' He got up and started running.

His brothers must have heard the sudden movement, because the calls got more frantic, and he started to hear running behind him. 'How the shell did they figure out where I went? No time to think of that now, gotta keep going..' His pace quickened. 'gotta keep going..'

Suddenly the voices sounded like they were getting closer. 'They might have seen me! need to get away and quick!' The voices continues to get louder.

"LEO! Come back here!" one called. Sounded like Donnie.

"Yea dude! What's going on?" That one sounded like Mikey

"Leo, why are you doing this? We just want to help you! Stop running for just a moment!" Then came the angry Raph. But, he didn't sound angry. Instead, he sounded concerned, and scared.

"Please! Just stay away from me! I don't want to hurt you!" Leo pleaded as he ran as fast as he could. "PLEASE!"

Suddenly, something wrapped around him and he fell to the ground. He looked down. It was the chain of Mikey's kusarigama chain. 'NO!' his head screamed. He pulled against the chain, but found it useless to struggle. They had got him. 'No can't give up, gotta get away! Gotta find a moment to escape'

His brothers ran up to him with Mikey holding on tight to the other end of the kusarigama. "Leo, what the shell is going on?" Raph exploded in his face.

Leo said nothing for a moment, then looked up to Raph and calmly replied. "I don't want to hurt you.." His eyes were expressionless. Then Raph noticed something in Leo's voice. It almost sounded Demonic. But, not enough for it to be heard clearly. "Please, just let me go. If not I will hurt you!" Raph heard it again. He also noticed that Leo's eyes were starting to glow red, and he was starting to tremble. He backed away.

"Raph, what's wrong?" Donnie said noticing that his older brother had backed away and had a glint of fear in his eyes. "Raph?"

"Let him go." Raph said not taking his eyes off of Leo's.

"What?" Donnie said Confused by what Raph had said.

"Let him go." Raph repeated. Leo's eyes were really starting to turn red, and look like he was in pain. His younger brothers weren't moving fast enough, and grabbed the kusarigama out of Mikey's hand and unwrapped Leo. He grabbed his brothers and backed up as he watched Leo get up and stare at him. Then he noticed something else; Leo actually had a tear running down his face. He tightened his grip on his brothers as Leo turned and ran down the tunnel.

Raph let go of his brothers as Leo disappeared into the darkness. He turned to face his brothers.

"What was that?" Donnie asked.

"Donnie, whatever was in that dart is somehow controlling Leo's mind. I think he's losing the battle that's going on in his head to keep his sanity, body, and mind. That's why he didn't want us to get him…" Raph concluded.

"How could you tell all that from just looking at him?" Donnie was still clearly a little confused.

"Yeah!" Mikey agreed.

"His eyes. His eyes were turning red. And he looked like he was in pain. His head must have hurt from the chemicals trying to take over him. Listen, the only reason why he suddenly ran off is because this transformation scared him a lot and he hasn't been able to fully keep himself under control." Raph explained. "So lets keep an eye on him, but don't harm him. Make sure he doesn't know you're there."

This brothers nodded. "come on guys, lets go home…" Raph turned and started to walk home with his brothers right with him.


	3. Chapter 3

Leo ran as far as his feet could take him until he ended up on a rooftop ten miles from his home. 'Yes. This dision was good. Safe brothers are happy brothers right?' He lost himself in his conversation with himself in his head and didn't even realize that he wasn't alone.

The blue banded turtle only had a little bit is sanity left in him and he knew he had to keep going. But he was snapped out of his thoughts when he heared someone drop behind him. He turned. It was Karai. "No Karai must leave. Not safe. Must go!" Leo said, voice getting a bit disorianted. "You leave. Not safe near me.."

"Oh hush you." She snapped at him. "Baxter wasn't joking. That chemical is working like a charm! Heh." She laughed and started to walk closer to Leo. "I'm guessing by the looks of it, your gonna lose control of yourself in the next day. Ah well. The Shredder would love to see you Leo. And I guess the only way to get you there is by dragging you there myself." She smiled.

"Karai not going to get me. Won't let you." He growled. He took his sword out and charged at her. He missed and stumbled a bit when landing. Just as he was going to turn around Karai smacked him hard in the face with the handle of her sword. He fell unconscious. Karai smiled at him.

"Take him back to The Shredder. He will be very pleased to see that we captured stockman's little experiment." The foot bots that here hiding in the shadows kept out and grabbed Leo and ran back to their hq.

Leo slowly regained consciousness and realists that he was in a cage. 'No. This can't happen. Must get out of here. Gotta find a way out..' He looked around but then realized that there was no escaping this room. It was full of foot bots and not to mention Fishface, Rahzar, Karai, and The Shredder himself.

"Looks like my Little pet has finally woken up from his nap..." Shredder mocked. "And pretty soon. Your mind will belong to me.. You see. Were the cause of this little accident and whatever stockman used in that dart, is going to be pretty hard to find an antidote for." He laughed. "Take him out of his cage." He ordered Rahzar. Rahzar moved and opened the cage. He took Leo out and Leo didn't even struggle. He knew that he has lost this battle and there was nothing he could do about it. "Looks like the Shredder wants to play with his new toy" Rahzar growled. He laughed and tossed the leader into the middle of the room.

"Now. You obey the Shredder. I am your master. Got it?" Shredder asked.

"Never." Leo replies in a slow, low voice. "I will never obey you."

"Haha. Yea we'll see about that." Shredder lounged at Leo. The turtle didn't have time to react before the Shredder punched him square in the chest. Leo flew back a few feet. "Who is your master?"

Leo fell silent. He was losing the war between sanity and insanity, and before he knew it. His eyes were red and lost control. "You are..." He grumbled.

The Shredder smirked. "Good. Who will you only listen to?" He asked Leo.

Leo paused for a moment then answered. "You are. Master Shredder..." Leo replied. By this time, Leo had gotten up and kneeled infront of the Shredder in respect of his "master".

"Good. Now tonight, I want you to destroy the turtles." Shredder ordered, handing him a new mask. It was black with dark blue tips.

Leo looked up and smiled. "Yes master." He said. He took the mask, took off his blue one, and put on the black one. "As you wish."

Shredder smiled and patted the boy on the shoulder. "Very good Leonardo. Very good."

***yes I know this was short. It was rushed last night because it was like 11:00 at night and I needed to go to bed. haha anyway, I hope you enjoyed. more will be soon hopefully.***


	4. Chapter 4

It's been about a week since the guys last saw their oldest brother, and they were starting to get worried about him. Lately, their normal life hasn't been what it used to be. Donatello has been in his lab most of the time trying to figure out what exactly happened with Leonardo. He never talks to his brothers as much anymore.

Michelangelo stopped pranking as much anymore. He rarely smiles either. He just spends most of his time in his room most of the time or in the kitchen making creations.

As for Raphael. He has been out topside every night looking for his brother. Ever since the incident happened, sensei gave the role of leader until the problem is resolved. Raph took the role without pleasure. He thought it didn't feel right when he was leader. It was Leo's part not his. He wasn't happy at all when he was given the role, but he decided that as soon as they had Leo back, and back to the way he was, he would give Leo the role back without question.

Now the guys were out on a patrol. Mostly looking for their missing brother than beat up bad guys. They hardly talked in a while and Raph thought it was nice to have this night to talk to his youngest brothers. He missed hanging out with them.

Raph turned to his brothers. "you guys see anything yet?"

"No. Not yet." Donnie replies, not taking his eyes off the buildings in front of him.

"Nope." Mikey says. You could clearly hear that he was depressed about the whole thing.

Raph sighed and went back to where he was looking before. It wasn't long before he heard a low growl coming from the shadows. He turned to the sound and saw a pair of glowing red eyes intensely staring at him. He backed up. Raph looked back at his younger brothers and saw that Mikey and Donnie were right next to him staring at the eyes as well. 'Guess they heard it too." Raph thought. "Alright, come out! We know you're there so why hide?" Raph said to the creature.

Slowly, the thing came out of the shadows and the three brothers stared in shock. The thing that came out of the shadows, was their older brother. only, Leonardo didn't have his normal blue mask and gear anymore. He was wearing a black mask with navy blue tips, and footgear. Leo growled.

"Leo… no…" Raph breathed. He started walking towards his brother, but then stopped when he heard Leo growl louder. "Come on Leo, snap out of it bro.. I know you can do it. " he reassured him. But it didn't work. Instead, Leo pulled one of his katana out and lunged at Raph.

Raph reacted quickly. He moved out of the way, and as soon as Leo landed, Raph took his brother's wrists, threw him over his shoulder and smacked him down onto the roof. There, Raph was holding Leo down with a hard grip. Raph looked at Leo. "I'm sorry bro, but you're acting way crazy."

Then, Leo finally spoke. "I am no brother to you turtles." he said in a low voice. Everyone looked at Leo in surprise and concern.

"What do you mean Leo?" Don asked.

"Master Shredder wants me to dispose of you mutants, and your sensei." Leo replied expressionless.

"What?! Shredder isn't your master! You live with us! the only master you have is Master Splinter! Come on bro! You don't believe that do you?" Mikey asked.

"Brothers? Hah. I have no brothers. You're lying to me and now you're going to pay.." Leo managed to get out of Raph's grasp and knock him down. Eyes glowing brighter, he grabbed his sword and charged at swiped at Donnie and missed. Don managed to wack him in the head with his bo, but it didn't seem to faze Leo for a second. Leo turned and punched the tall turtle square in the chest, knocking the wind right out of him. The oldest turned to the youngest. The black clad turtle ran up to Mikey and slammed him into the wall. Sword in hand, he was about to make his blow.

"That is enough Leonardo. Come, the Shredder must say something to you." It was Karai.

Leo let go of his brother and faced the guys. "Count that as a warning. Next time you will not be as lucky." he growled. He ran up to Karai then threw a smoke bomb before Raph could grab him. Once the smoke cleared, they were gone.

Raph stared at where the duo disappeared. Then heard Donnie. "Mikey, are you ok?" Donnie asked by Mike's side.

"Yeah Donnie. I'm fine. He didn't hurt me." Mikey replied giving his brother a reassuring smile.

"Alright. Raph, come on. Let's go." Donnie said to his red clad brother.

"Yeah.. I'm coming.." Raph said taking one last look at the spot, then following his brother's back to the lair.

* * *

Leo kneeled in front of the Shredder. "You wanted to speak to me Master?" Leo asked.

Shredder walked up to Leo. "Yes. It seems that over the week you have showed an amazing amount of loyalty to me. Especially tonight. You have fought your brothers with no mercy. You wouldn't have done that if you weren't that loyal. So, I am giving you the spot of 3rd in command. Now, If you continue to show this much loyalty, there will be an award bestowed onto you."

Leo looked up to Shredder. "Thank you Master. I will not fail you." Leo promised.

"I know you won't" Shredder smiled. "You may go. Get some rest. It's going to be another long day tomorrow."

"Yes master." With that, Leo got up and left the room to his own. 'I will get you turtles if it's the last damn thing I do.' He thought to himself.


End file.
